The operation of fluorescent and other gas discharge lamps and lamp systems for display lighting, large advertising signs, and the like by means of conventional power systems has many disadvantages. These include the need for expensive starting systems, the need for current limiting devices such as massive ballast, because of the negative conductive characteristics of gas discharge lamps, to prevent damage to lamps and other circuit components, etc., as is well known. Heavy currents may flow if not prevented and lamp systems of this type require protection against such after they have been started. Numerous proposals have been made in the past to solve these problems or prevent them from arising. Such proposals include schemes to operate the lamps by alternating current at much higher than normal commercial frequency, to reduce or eliminate the need for ballast. The use of special starting devices has been proposed in many instances, as in U.S. Pat. No. 3,500,125, which suggests use of high starting voltage for each cycle, while employing conventional power at 50 cycles or so. U.S. Pat. No. 3,471,747 suggests a system having a high voltage starter which is cut out for normal operation. In U.S. Pat. No. 3,354,350, it has been proposed to use multi-vibrators to break up conventional AC sine waves into high frequency pulses of approximately square wave form, employing such semi-conductor devices as Zener diodes and transistors to divide the wave forms, etc. The purpose was to produce a much higher than normal frequency, which is also a purpose of the present invention, but the means used and the components involved are much more complex than in the present case. Other prior art investigators, as in U.S. Pat. No. 3,263,122, have suggested that one start initially with direct current, converting it by means of switching devices, including semi-conductor devices, to an alternating current of suitable high frequency. The amplitude of current in that case would be limited by special means to prevent damage to the lamps and other circuit components.
Still other proposals have included the use of specially configured transformers (U.S. Pat. Nos. 3,350,605 and 3,263,122) and various simple or more complex systems as in U.S. Pat. Nos. 3,368,107 and 3,639,827. In many of the devices of the prior art, semi-conductor elements of various types are used as triggering means, current limiting means and the like. Intermittent flashing means forcertain lights in combination with continuous power supply to others is suggested in Skirvin, U.S. Pat. No. 3,585,483.
The present invention is particularly concerned with improvements in apparatus and method for supplying power to light large illuminated signs to brightness which varies with ambient light conditions, although it is not limited to such use.
As far as applicant is aware, none of the devices or arrangements of the prior art are adapted for or suitable to use for the combination of the following functions, namely, (a) converting commercial AC power to high frequency AC of suitable wave form, (b) providing automatically for continuous readable illumination at minimum power consumption, brightness being variable with the ambient light as needed, and (c) interrupting and restarting the illumination periodically and repeatedly, when needed, as for flashing. The present invention has the accomplishment of these functions in combination as its major objective and a further objective is to do it all with simple and relatively inexpensive circuit components which are highly reliable.
A particular object of the present invention is to obtain essentially square wave DC pulse configuration at an optimum or near optimum high frequency which when converted back to AC will substantially eliminate the need for conventional ballast, i.e., to cut down size and weight of ballast required. Another object is to maintain control automatically over the width of pulses applied to the load, in response to ambient light conditions so as both to conserve power and to provide improved readability in display lighting. Many signs of this type are illuminated continuously at excessive brightness levels under ambient darkness conditions while often being inadequate when the ambient light is greater, as at twilight or in bright moonlight.
A further object is to make the sign capable optionally of intermittent or "flashing" operation, without substantially adding to the cost or complexity of the circuit components.
It has been proposed in the prior art to control or vary the brightness of lighting, by varying the timing of on and off portions of the power cycles in high frequency power systems. One such system is described in U.S. Pat. No. 3,619,716. However, the circuitry components therein required are numerous and complex and a further object of the present invention is to accomplish the desired result in a superior manner and with much simpler and less expensive equipment. The prior art apparatus mentioned, for example, requires multiple transformers, which are not needed in the present system.
Further objects of the invention are to operate banks of lamps, as in large display signs of illuminated type, with reduced overall power requirements, as compared with systems of the prior art. Light intensity is adjusted manually, automatically, or both, not only to reduce power consumption but also to improve readability under ambient light conditions. This may be accomplished preferably by use of simple and well known types of light-sensitive devices. Ambient light often varies widely from hour to hour as well as from day to day, so effective control over the power applied is important in reducing operating costs.
The foregoing and other objects will be more fully understood and appreciated by reference to a detailed description of a preferred embodiment which follows: